Production of active glycosylation-deficient γ-secretase complex for crystallization studies

Generation of Flp-intm-ready DG44 and Lec 3.2.8.1 CHO cell lines for quick and easy constitutive protein expression

The well-characterized cell line Chinese hamster ovary (CHO) has been used to produce numerous biopharmaceuticals and is an important tool for basic research. However, introducing foreign DNA into specially modified CHO cells such as DG44 and Lec 3.2.8.1 can sometimes be an arduous process. Here we show that the Flp-intm plasmid can be modified to produce a fluorescent tracer protein tag (mCherrytm) as a fusion reporter, to allow for the rapid selection of single-cell sorted, isogenic Flp-intm-ready DG44 and Lec 3.2.8.1 cell lines. These two cell lines are stable and viable and may be useful for applications such as antibody production and crystallographic studies. Here we provide key details on how the modified pFRT/CherryZeo plasmid may be used to incorporate Flp-intm technology into virtually any desired target cell line in a fast, safe and reliable manner.

Glycosylation status of nicastrin influences catalytic activity and substrate preference of γ-secretase

γ-Secretase complex, the assembly of nicastrin (NCT), Presenilin (PS), Presenilin Enhancer-2 (PEN-2) and Anterior pharynx defective 1 (Aph-1), catalyzes the cleavage of amyloid precursor protein to generate amyloid-β protein (Aβ), the main culprit of Alzheimer's disease. NCT becomes matured through complex glycosylation and play important role in γ-secretase activity by interacting with catalytic subunit PS. However, the role of NCT glycosylation on γ-secretase activity and substrate specificity is still unknown. The purpose of this study is to investigate the effect of NCT glycosylation on γ-secretase activity and substrate specificity in a group of glycosylation mutant lectin resistant CHO (Lec) cells. CHO Lec-1 cells lack glycosyltransferase-I, GnT-I, thus N-glycan on NCT are all oligomannose type, whereas CHO Lec-2 cells synthesize NCT containing sialic acid deficient oligosaccharides due to the impairment of cytidine 5'-monophosphate-sialic acid transporter. Here, we reported that mutant CHO Lec-1 and Lec-2 reduced γ-secretase activity in both cell-based and biochemical assays, and that CHO Lec-1 preferentially reduced Aβ generation. Endogenous level of γ-secretase complex, subcellular distribution of γ-secretase subunits and the level of functional γ-secretase complex remained unchanged in mutants. Interestingly, Coimmunoprecipitation study revealed that mutant γ-secretase could recognize substrate as well as parental γ-secretase. Our data suggests that thorough glycosylation of NCT is critical for enzymatic activity and substrate preference of γ-secretase.